1. Field of the Invention
The present invention relates to a position control system. More particularly, it relates to a system for position-control of a head in an information storage system, for example, an optical disc system, through a stepping motor.
2. Description of the Related Art
In an information storage system such as an optical disc system, an optical magnetic disc system, or a magnetic disc system, a position of a head therein is controlled in a seek mode and, thereafter, in a tracking mode. In the seek mode, the head is moved to a portion adjacent to a target track in a rotatable disc at a high speed. Thereafter, in the tracking mode, the position of the head is finely controlled at the target track. Due to the requirements for a larger storage capacity, the number of tracks in a disc has greatly increased, and a distance (pitch) between the adjacent tracks has been made very much shorter, for example, the distance between tracks of an optical disc in an optical disc system is now approximately 1 to 2 .mu.m. Accordingly, a very precise positioning control is required.
In an information storage system, for example, in an optical disc system, a track servo system is provided. The track servo system includes a DC motor for moving an object lens for focusing a light beam onto the track. The DC motor is mounted on an optical head, and a track servo unit drives the DC motor. In the tracking mode, the track servo system adjusts a positional error of the optical head. If the positional error exceeds a limit controllable by the track servo system, a second motor other than the DC motor mounted on the optical head, used for the seek operation and for moving the optical head, can be driven to position the optical head outside the limit controllable by the track servo system. Namely, in the tracking mode, control of the head position is carried out by a track follow-up (tracking) control using the DC motor mounted on the optical head and by a compensating control using the second motor installed outside of the optical head. In other words, the second motor is used for both the seek operation and the tracking operation.
In a prior art optical disc system, the above-mentioned second motor is a DC motor, for example, a voice coil motor. In an optical disc system using the voice coil motor, the optical head can be continuously moved, thus realizing a continuous follow-up (tracking) control of the optical head to a plurality of tracks. Particularly, when the tracks are spirally and continuously formed, a continuous read or write of data can be achieved. However, the voice coil motor requires a closed loop control system, necessitating a complex circuit construction, and in addition, the magnetic circuitry is large, resulting in low efficiency.
To overcome the above defects, another prior, art optical disc system has proposed using a stepping motor as the second motor. This stepping motor operates with high efficiency and can be controlled by an open loop control, i.e., does not require a closed loop control, reducing the total cost of a system. The stepping motor can be used for both the seek operation and the tracking operation, and to minimize the seek time, the stepping motor is driven under a maximum current. In this prior art optical disc system, the maximum current is still supplied to the stepping motor, even in the tracking operation mode, to rotate the stepping motor by one step so that a positional error is compensated. However, this maximum current drive of the stepping motor causes the optical head to be sharply accelerated, and as a result, the object lens and the voice coil motor, both mounted on the optical head, are sharply accelerated. This sharp acceleration disturbs the track servo system, bringing the track servo system outside of the controllable range, and accordingly, rendering the track servo system temporarily inoperative. Consequently, a continuous read or write operation for a plurality of tracks cannot be achieved.